2194 J. Electrochem. Soc.: E L E C T R O C H E M I C A L SCIENCE A N D T E C H N O L O G Y October 1982

icas--1980," Vol. 1, Mater ia ls Recovery and Ut i l - izatior~---MD-1, I. LeMay, Editor, pp. 67-71,

ASME, New York (1980). 11. R. B. Swaroop, C. W. Boquist, and J. E, Battles,

Abs t rac t 52, p. 145, The Elect rochemical Society Extended Abst rac ts , Pi t t sburgh, Pennsylvania , Oct. 15-20, 1978.

12. T. W. Olszanski and H. Shimotake, Abs t rac t 53, p. 148, The Elect rochemical Society Extended Ab- stracts, P i t t sburgh, Pennsylvania , Oct. 15-20, 1978.

13. G. Bandyopadhyay , J. T. Dusek, and T. M. Galvin, Ceramurgia Int., 5, 95 (1979).

14. J. E. Battles, F. C. Mrazek, and N. C. Otto, Argonne Nat ional Labora to ry Repor t ANL-80-130 (De- cember 1980).

15. J. G. Eberhar t , Argonne Nat ional Labora to ry Re- por t ANL-79-34 (August 1979).

16. J. E. Ebe rha r t and J. E. Battles, U.S. Pat. 4,189,827 (1980).

17. J. Buchanan etal., (p. 32), B. A. Askew (pp. 42- 43), and S. Sudar, L. R. McCoy, and R. C. Saun- ders (pp. 177-190), Argonne Nat ional Labora to ry Re.port ANL-80-128 ( F e b r u a r y 1981).

18. A. A. Chilenskas, J. C. Schaefer , W. L. Towle, and D. L. Barney, Argonne Nat ional Labora to ry Re- por t ANL-79-59 ( J a n u a r y 1980).

19. G. Bandyopadhyay , This Journal, 128, 2545 (1981). 20. F. J. Mar t ino etal., in "Ba t te ry Design and Opt imi -

zation," S. Gross, Editor, p. 425, The Elec t ro-

chemical Society Sof tbound Proceedings Series, Princeton, NJ (1979).

21. I. L. Romanova and J, A. Seli tskii , Etectrokh~miya, 6, 1776 (1970).

22. M. J. Lysaght , in "Ba t t e ry Separators ," The Colum- bus Section of The Elect rochemical Society, Co- lumbus, Ohio, F e b r u a r y 18-19, 1970, pp. 57-73.

23. J. A. Orsino, E. J. Dunn, Jr., and W. S. Bundy, p re - sented at The Elect r ica l Insula t ion Conference on Mater ia ls and Appl icat ion, Washington, D.C., Feb. 21, 1962, and Publ ica t ion No. 253-62, Na- t ional Lead Company Research Labora tory , Brooklyn, NY.

24. J. A. Orsino and E. J. Dunn, Jr. , p resented at The 1961 SAE In te rna t iona l Congress and Exposit~bn of Automot ive Engineer ing, Detroit , Michigan, J a n u a r y 9-13, 1961.

25. H. Ohno and H. Shimotake, Abs t rac t 80, p. 215, The Elect rochemical Society Ex tended Abstracts , Hollywood, Flor ida , Oct. 5-10, 1980.

26. A. Scheidegger , "The Physics of F low Through Porous Media," p. 26, Univers i ty of Toronto Press, Toronto (1974).

27. J. J. Lander , in "Ba t t e ry Separa tors ," the Columbus Section of The Elect rochemical Society, Colum- bus," Ohio, F e b r u a r y 18-19, 1970, pp. 4-24.

28. H. Shimotake, Personal communicat ion. 29. R. A. Sha rma and T. G. Bradley, Abs t rac t 148, p.

381, The Elect rochemical Socie ty Extended A b - stracts, Los Angeles, California, Oct. 14-19, 1979.

Effect of SO, and pH on the Anodic Behavior and Pitting Corrosion of Alloy 600

Ph. Berge

Electricit~ de France, Service de la Production Thermique, Groupe des Laboratories, 21, all~e Privee-Carrefour PIeyel, 93206 Saint-Denis Codex, France

V. Jovancicevic

Groupe de Recherche No. 4 du C.N.R.S. Physique des Liquides et EIectrochimie, Universit~ Pierre et Marie Curie, 4, PLace Jussieu, Paris Codex 05, France

and D. Noel and P. Saint-Paul

Electricit~ de France, Direction des Etudes et Recherches, D~partement Etude des Mat~riaux, Centre des Renardieres, BP No. 1, 77250 Moret-sur-Loing, France

ABSTRACT

The anodic behavior and pitting corrosion of Alloy 600 in solutions containing low concentrations of SO~ and H~SO~ were studied by cyclic vol tammetry near the activation peak. The results are presented in the form of an [H2SO4] vs. [SOztot] corrosion diagz'am for different sweep rates. A model based on the competi t ive adsorpt ion of SO2 and HSO3 is proposed. The effect o fpH on the various adsorpt ion states is discussed. The adsorption of HSO3- anions gives rise to the breakdown of the oxide film and that of SO2 provides a protective effect; the adsorption of SO2 depends very strongly on the pH. Two types of pit t ing were observed at different potentials in the activation range. Relatively few and deep pits occur at low HSO~- concentrations (-0.47 V/SSE) while numerous and shallow pits develop at higher concentrations (-0.52 V/SSE).

Recently, p i t t ing corrosion observed dur ing s torage of AHoy 600 tubes for s team genera tors of pressur ized wate r reactors was a t t r ibu ted to the presence of oxi- dized sul fur species (SO2, SO42-, HSOa- , . . .) and humidi ty . The role of SO2 on a tmospher ic corrosion was recognized ve ry ear ly (1) and s tudied by severa l authors (2-5). Low al loyed steels rust in these a tmo- spheres and the corrosion products contain main ly sul- fates. On the o ther hand, s tainless steels exposed for long t imes to indus t r ia l a tmospheres can undergo p i t - t ing corrosion (6). The effect of SO2 on anodic behavior of F e - C r - N i al loys was s tudied in sulfuric acid solu- tions and the increas ing of the ra te of active dissolu-

Key words: SO=, oxide film, adsorption, pitting corrosion.

t ion was observed (7, 8). Pou i l l a rd and Chiola (9) have s tudied fer r i t ic and austeni t ie s tainless steels in solutions containing chlor ide and sulfur dioxide. These authors a t t r ibu ted the accelerat ion of p i t t ing to cathodic depolar iza t ion in the presence of SO2.

Elec t rochemical studies of p i t t ing corrosion of pas- s ivable mate r ia l s in ha l ide-conta in ing solutions ,~how the occurrence of pits above a cer ta in cri t ical potential , named p i t t ing potential . Below this potent ia l or in the absence of halide, few cases of p i t t ing corrosion have been observed. For example , Ogura and Takesue (10) s tudied the format ion of pits at cathodic potent ia ls on passive i ron in chlor ide solutions. I j ze rmans and Van der Krogt observed pits on Type 316 stainless steel in

) unless CC License in place (see abstract).  ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 130.203.136.75Downloaded on 2016-09-17 to IP

Vol. 129, No. 10 ANODIC BEHAVIOR 2195

a solution of H2S (11) and also in NaSCN (12). Szklarska-Smialowska (13) obtained pits on iron in Na~SO4 solution close to the passivation potential.

In sulfuric acid solutions with SO2, some funda- menta l studies concerning oxidation, adsorption, and reduction of SO2 have been conducted, main ly on plat- inum (14-16). It should be noted that in these studies, H2SO4 is either a support ing electrolyte, or a solution used for pre-electrochemical t rea tment of the elec- trode and apparent ly does not modify the phenomena ~" under study.

This work is related to the study of pi t t ing and gen- ~. eral corrosion of Alloy 600 in solutions containing SO2 ~ o and H2SO4, in the absence of chloride, near the activa- tion peak by cyclic vol tammetry. The respective effects of HSOs,-, SO2, and pH on the corrosion of this alloy have been studied. -0,7

Exper imenta l Cond i t ions A conventional apparatus for cyclic vol tammetry

including a potentiostat supplied with a t r iangular signal generator and an XY recorder was used.

The working electrode was machined from a tube of Alloy 600, cold-pilgered and mill annealed at a tem- perature close to 980~ The composition of this alloy is shown on Table I. Small sections of tube, 19 mm in di- ameter, are coated with a resin in order to obtain, on the outside surface, a working area of 1 cm ~. The elec- trical contact was ensured by an embedded rod screwed into the tube. The working electrode was me- chanical ly polished with 600 grit paper just before being immersed in the solution. A saturated mercury- lO mercurous sulfate electrode was used as a reference and the counterelectrode was made of plat inum.

The SO2-I-~2804 solutions, with various concentrat ion <~ ratios varying from 5 �9 10 -5 to 9 �9 10-1M in SO2 and ~ o from 10 -8 to 10-1M in H2SO4, were prepared using

Z r e a g e n t g r a d e chemicals and demineral ized water.

Results and Discussion ~- -~o

Ef]ect of SO2.--The electrochemical behavior of Al- loy 600 was first studied in solutions containing only ~o SO2. Figure 1 shows the corresponding mixed poten- tials as a funct ion of the total concentrat ion of SO~. -20 Three potent ial levels, --0.58, --0.7, and --0.615 V/SCE and abrupt t ransi t ions between these levels can be noted. Cyclic vol tammetr ic curves were obtained for concentrat ions corresponding to each of the three levels (5 . 10 -1 , 102 , and 5 - 1 0 - ~ M ) in a potential range from --0.56 to --0.3 V/SSE, which is anodic with respect to the mixed potentials 1 (Fig. 2). Star t ing from mixed potential, the vol tammetr ic curves were l imited to the potential range, after repetit ive cycling (5 mV/sec) , corresponding to the activation of Alloy 600. The shift of the mixed potential for 5 �9 10 -~ and 10-2M SO2 to anodic values can be related to the modi- fication of the surface oxide film du r ing the scanning cycle. For the 10-2M SO2, a significant current appears in this potential range.

For comparison, on plat inum, in the same potential range, the predominant process is the adsorption of SO2 (14-16).~ The oxidation of SO2 to sulfuric acid and its reduction to sulfur occur at potentials higher than --0.2 V/SSE and less than --0.6 V/SSE, respectively. The curve obtained for p la t inum is shown in Fig. 3.

Cyclic vol tammetr ic curves are not compensated for ohmic drop. Even in SOe solutions at low concentrations w h e r e the re- s istance of the solution is very large (100 9 - cm - 10 K ~2 �9 era) the current densi t ies remain small and ohmic drop is not beyond 0.05V).

~According to Contractor and Hira Lal (16) the adsorption process may be related to the formation of more than a mono- layer of intermediate species or adsorbed sulfur.

-0 ,6

I 10"4

SO 2

l q i 10"3 I0-2 10 4 10 0

TOTAL CONCENTRATION OF SO2, C (M)

Fig. 1. Mixed potentials for Alloy 600 in SO~. solutions as a func- tion of the total S02. concentration.

I I I

SO 2

. . . . . . . . ............... ::"..::::'

~ / / / / / 1 . / / 1 1

/ . ~ ' ~ 5 10-1M

Z J " ___

. . . . . . . 5.10-4 M

I I I -0.6 -04 -0.2

POTENTIAL, (V/s S E')

Fig. 2. Cyclic voltammetric curves obtained for S02 concentra- tions corresponding to the three equilibrium potential levels; sweep rate 5 mV/sec.

In analyzing these results, where adsorption phe- nomena play a major role, the chemical composition of the solution as a funct ion of the total concentrat ion of SO2 is an impor tant factor. When the actual concen- trations of SO2 and HSO3- as well as the pH of the solution, calculated using the equi l ibr ium constant for the first dissociation of sulfurous acid (17)

I-I2SO8 ---- H + + HSO3- PKa : 1.97 [1]

are plotted as a funct ion of the total concentrat ion of SO2 as shown in Fig. 4, it can be noted that the three mixed potent ial levels correspond to three different regions. The first one is characterized by actual con- centrat ions of SO2 and HSO3- less than 10-3M and a high pH; the second one by higher concentrations, with predominance of HSO3- and an intermediate pH of 2-3; the third one by even higher concentrations, with predominance of SO2 and a low pH.

Table I. Chemical composition of the Alloy 600 material studied (weight percent)

C S Si Mn Ni Cr Co Ti Cu A1 Mo Fe Zr Mg P

0.032 0.001 0.07 0.20 75.6 14.72 0.04 0.18 0.33 0.19 0.10 8.50 0.001 0.026 0.009

) unless CC License in place (see abstract).  ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 130.203.136.75Downloaded on 2016-09-17 to IP

2196 J. FAectrochem. Soc.: ELECTROCHEMICAL SCIENCE AND TECHNOLOGY October I982

i

I i i i -o.~ o,o o,s 1.o

POTENTIAL (V/SSE I

Fig. 3. Cyclic voltammetric curve for platinum in a 10-2M solu- tion of SO2 at a sweep rate of 5 mV/sec; SO2 is adsorbed from --0.2 to --0.6 V/SSE.

It is in teres t ing to compare these results with those shown in Fig. 5 obtained on Alloy 600 in solutions of H2SO4 in the absence of SOs, and in the same range of pH. One notes the presence of a low activation peak, the in tensi ty of which rises as the pH decreases, fol- lowed by passivation.

Effect of the pH.-- By slow oxidation of SO~ in the presence of oxygen, sulfuric acid can be formed, ac- cording to the following reaction

SO2 + H~O + 1/20~ ~ H2SO4 [2]

and its presence can modify completely the electro- chemical behavior of Alloy 600.

Figure 6 presents the cyclic vol tammetr ic curves ob- tained for Alloy 600 in a solution containing 2 �9 10-3M SO~ and 2 �9 10-2M H~SO4 that show an impor tant ac- t ivat ion peak. This peak increased for each successive sweep unt i l a s tat ionary value of a few mA/cm '2 after

ioo

-r

<

0

W 0

b

I I I i I

SO2

2

TOTAL CONCENTRATION O F SO 2 , ( M )

Fig. 4. pH and actual concentrations of SOs and HSO3- as a function of the total SO2 concentration.

10

E

<

P

b-. Z # 0.0 w"

0

I I

H2SO4

\

I I I I -0.6 -0.4 -0.2 POTENTIAL ,(V//sSE)

Fig. 5. Cyclic voltammetric curves for Alloy 600 in H2SO4 of various concentrations; sweep rate 5 mV/sec.

1 5 0

E < =~joo

Z W D

Z W n'-

~ 5o 0

L

SO2+H2SO 4

!+2.1{~2M H2SO 4 so4 2

0.5 POTENT~r Q3

Fig. 6. Cyclic voltommetric curves for Alloy 600 in solutions of 2 . 1 0 - ~ M SO2, 2 . 1 0 - ~ M H2SO4, and 2 . 1 0 - 3 M SOs + 2" 10-2M H2SO4; sweep rate 5 mV/sec. The circled numbers indicate the order of the successive sweeps.

about ten sweeps had been reached. In solutions con- ta ining only one of the two species at the same con- centration, practically no activation peak is observed.

This effect has been studied as a funct ion of the rela- tive concentrations of SO2 and H2SO4, in solutions ob- tained by mixing pure solutions of SO,, and H2SO4 and

) unless CC License in place (see abstract).  ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 130.203.136.75Downloaded on 2016-09-17 to IP

Vol. 129, No. 10 ANODIC BEHAVIOR 2197

t 802 + H 2 S O 4

t 4

* ~ * * * * * * * * * /

~ GENERAL / I / / OORRO=ON + / *

. I / A . , . § �9 . + /

i l O V ~ + .5 mV/se�9 + . " . * + - + ~ / . . . . . . . .

. l § �9

10-3 , ! * . , , / / /

t

, i i I I

lO-4 lO-3 lO-2 lO -1 TOTAL CONCENTRATION O F S0 2 (M)

it, a potential range from --0.56 to --0.3 V/SSE for three different sweep rates: 5, 1, and 0.2 mV/sec. The results show that an activation peak occurs in certain concentrat ion ranges only. The passivation and corro- sion ranges have been determined, taking as a criterion the absence or presence of a 10 #A/cm 2 peak. Respec- tively, the results obtained are shown in Fig. 7 in the form of a diagram with two predominant domains: the one above the curve corresponds to general corro- sion and the one below the curve to passivity. The l imit between these two areas depends on the sweep rate. Fur thermore, an in termediate area (hatched band) could be defined, in which the surface of the electrode is pitted after several scanning cycles. The t ransi t ion between the corrosion area and the passivity area is fair ly well defined, but the t ransi t ion between the pi t t ing area and the general corrosion area is much broader.

The diagram in Fig. 7 was drawn as a function of the lo c total concentrat ion of SO2 and H2SO~. However, since the SO2 content is in equi l ibr ium with the HSOs- and H + contents, the total concentrat ion of SO2 does not ~ correspond to the actual concentration, which varies as ~1o a function of pH (Eq. [1]). For this reason, the pH variations and the actual concentrations of SO2 and

plotted in Fig. 8 as a funct ion of the ~d H S Q - have been total concentrat ion of SO2 for those ini t ial SO2/H2SO4 co

concentrat ion ratios corresponding to the boundary between the two main areas showed in Fig. 7. Values o163 corresponding to a sweep rate of 5 mV/sec have been chosen.

Using these results, the corrosion diagram was re- ~ / calculated as a funct ion of the actual concentrat ion of ~1~4

o S02 SO2 to give Fig. 9. This figure shows the pH variat ion ~ / along the l ine separat ing the two areas and the cor- 3 responding variat ion of the mixed potential for Alloy <OlO4 600. The mixed potential curve and the corrosion dia- grams are very similar. Fur thermore, the max imum in pH corresponds to a m i n i m u m in the mixed potential. For high SO2 concentrations, a very sudden potential ~6 Hs increase is observed while the pH decreases gradually, i

Oxide film and adsorption modeL--All these results make it possible to propose the following model: When the actual H S Q - concentrat ion is less than 10-3M (pH > 3), the Alloy 600 behavior is characterized by the formation of a protective oxide film independent

Fig. 7. Corrosion diagram as a function of the total concentra- tion of SO2 and of H2SO4 ob- tained by potential sweeping from --0.56 to --0.3 V/SSE and for various sweep rates. The dis- tinction between the corrosion and passivity areas is the pres- ence or absence of activation peak of 10 #A/cm 2. Crosses in- dicate data points.

I

10 0

of the total SO2 concentration. For higher concentra- tions of HSO:3- when pH ~ 3 the adsorption of HSO~- is presumed to occur (probably similar to that occur- r ing in solutions containing C1- anions) which de- creases the protective character of the oxide film.

The three mixed potential levels of Alloy 600 in SO,, (Fig. 1) can thus be exp]ained in the following way: (i) the first level (--0.58 V/SSE) at low total SO2 cor- responds to the presence of an oxide film; this film is very little affected by the small concentrat ion of the species present in the solution; (ii) the second level corresponds to an H S Q - concentration, higher than that of SO2. We then saw a potential drop (--0.7 V/ SSE) indicat ing a decrease of the protection of oxide

I ~ I I I

S02+ H 2 SO 4

pH

I I I I 10-',~ 1C~2 1(~ 1 100

TOTAL CONCENTRATION OF S02.(M )

Fig. 8. pH and actual concentration of SO2 and HSO3- corre- sponding to conditions along the line between corrosion and pas- sivity in Fig. 7.

) unless CC License in place (see abstract).  ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 130.203.136.75Downloaded on 2016-09-17 to IP

2198 J. Elec trochem. Soc.: ELECTROCHEMICAL SCIENCE AND TECHNOLOGY October 1982

J co

z o_

1() 2 LJ

�9 0

/ " ~ - 0 5 c)

_< +- -0.6 -

m~--O.7

I I I I

SO2+ H2SO 4

I

b) I , , I I I

\ \

N \ \ \

\ \

\ \ \ \ \

% i I I I I

I I / I I 1C~ 4 l d "3 10-2 1~ 1 10 0

A C T U A L C O N C E N T R A T I O N OF SO 2 ,~M~

Fig. 9. Concentration of H2S04 (a), pH (b), and equilibrium po- tent ia l (c) as a function of the real S02 concentration, correspond- ing to the line between corrosion and passivity areas. The dashed curve (---) in (b) represents the pH of the solution of S02 alone; sweep rate 5 mV/sec.

film, probably by the adsorption of HSO3- at a pH < 3; and (iii) the third level (--0.615 V/SSE), ob- served when the SO2 concentrat ion begins to predom-

inate over the HSO~- concentrat ion corresponds to the formation of a protective film of SO2.

This protective effect of SO2 and aggressive effect of HSOs-, on the one hand, and the effect of the pH on the adsorption film of SO e, on the other hand, can be demonstrated in solutions of SO2 containing small quanti t ies of H2SO4, by means of the cyclic vol tam- metric curves obtained for concentrat ions correspond- ing to various points of the corrosion diagram as shown in Fig. 10.

For total SO2 concentrations greater than 5 �9 10-~M, the SO2 adsorption film is characterized by the ex- istence of a cathodic loop in the vol tammetr ic curve (Fig. 10A, B, and C). At point A, the SO2 adsorption film formed at cathodic potentials gives better protec- tion to the metal surface (forward curve, no activity peak) than the anodic oxide film formed at potentials higher than --0.4V (backward curve, activity peak), probably preceded by a desorption of the SO2. Under these concentration~ and sweep rate conditions, the oxide is practically dissolved and the SO2 adsorption film probably reaches a thickness of several monolay- ers. When the respective concentrations of HSO~- and SO2 are reduced (curves B and C), the dissolution of the oxide film shifts toward cathodic potentials and the activation peak appears and increases dur ing the for- ward scanning; consequently, the area comprised by the loop is reduced, which indicates that there is less protection by SO2.

Point D corresponds to the complete disappearance of the loop for a total SO2 concentrat ion of 2 �9 10-2M where [SO2] = [HSOa-] . At point E, in the general corrosion area, where the HSO3- concentrat ion is greater than the SO2 concentration, a very substant ial activation peak can be seen; while at point F, where the oxide is stable enough and the HSO~- concentra- tion is very low, the activation peak is weaker in spite of the high acidity.

The corrosion diagram shown on Fig. 7 is easily ex- plained by the same reasoning along the l imit between passivity and corrosion. Three different behaviors can be distinguished as a function of the total SO2 concen- trat ion: (i) at low concentrat ions a sudden increase in the relat ive H S Q - concentration, while the actual SO2 concentration, still greater than the HSOz- con- centration, is practically constant, is accompanied by decreasing the acidity of the solution (Fig. 8). These conditions correspond to an oxide film increasingly affected by adsorption of HSO3- anions at low pH; (ii) at intermediate concentrations, where the relative HSO~- concentrat ion has almost the same value as the SO2 concentrat ion and where the pH remains fairly constant around 2, the film formed is an oxide film

25

E 25

<

Fig, 10. Cyclic voltammetric carves obtained at various points ~ c on the corrosion diagram; sweep rata ! mV/sec.

~: 75 LU fl:

3 50

25

@

-O.5

| ' I i

I i I I -0.4 -0.3 , , -O.5 -0.4 -01.3 PO~ENT,AL, (V/ssE)

50

25 C7

o <

- ~ ~0~- F-- - -

- so ~ ~ , Z �9

n- o

0 0

25

-0

lm V,,,~ec

I ~

I 1o4

A

DY I I I I

i~3 1o-2 10-I 1oO[ TOTAL CONCENTRATION OF SO~(M3 I

) unless CC License in place (see abstract).  ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 130.203.136.75Downloaded on 2016-09-17 to IP

Vol. 129, No. 10 ANODIC BEHAVIOR 2199

strongly affected by the HSO3- adsorption; (iii) at high concentrations, where the SO2 concentrat ion pre- dominates again in the solution, a decrease in pH oc- curs as the SO2 concentrat ion increases. The SO2 ad- sorption film is not affected, except when the pH be- comes very low.

This model also explains the two types of pi t t ing corrosion which are observed on Alloy 600 in the ac- t ivation area dur ing repeti t ive cycling.

Types of pitting observed.--The pit t ing corrosion area shown hatched in Fig. 7 consists of two parts: the first (region 1) is characterized by low concentrat ions of HSO3- (10-6-10-4M) and a constant concentrat ion of SO2 of about 10-4M, and the second one (region 2) by approximate ly equal concentrat ions of H S Q - and SO2, from 10 -4 to 10-2M.

Figure 11 shows the influence of the sweep rate and the total SO2 concentrat ion on the shape of the cyc,lic vol tammetr ic curves. In region 1, a peak is observed at approximate ly --0.47 V/SSE, the in tens i ty of which is practically independent of the sweep rate; while in region 2, a peak appears at --0.52 V/SSE whose in - tensi ty depends on the sweeping rate. In the in ter - mediate range, both peaks are observed at the same time.

These two peaks, separated by 0.05V, correspond to different morphological appearances of the pits formed.

Region 1 seems to be associated with a low density of deep pits (,~10 ~m) and region 2 with numerous, shallow pits (Fig. 12). The differences between the two types of pits may be due, on the one hand, to different stabilities of the oxide film in the two regions and, on the other hand, to the different sites of adsorp- t ion and breakdown of the film by HSOs- , depending on whether the concentrat ion of HSO3- is less than or greater than the SO2 concentrations.

The pi t t ing in region 1, corresponding to the most anodic peak, is obtained in solutions containing a small amount of HSO3-. Consequent ly , the oxide is fair ly stable, and the number of ini t iat ion sites is probably reduced.

In region 2, the HSO3- concentrat ion is high, and the n u m b e r of HSO3 adsorption sites increases, but the presence of SO2 at a concentrat ion close to the H S Q - concentrat ion probably prevents fur ther development of pits.

It should be noted that these pits were obtained in the activation range through successive potential cycles. If the potential is constant ly imposed at --0.47 V/SSE, the current decreases rapidly to a few ffA/ cm 2, which corresponds to passivity. On the other hand, if the potential is imposed at --0.52 V/SSE (region 2), the current increases to a few tenths of ~A/cm 2 and pits continue to develop.

50

50

E <

o

100

11(# M SO 2 5.152 M H 2 SO 4

/ 2 ( - - ~ _ ~ - - - 5mV/sec

..,'" '-,, :' ". 2.104 M SO 2

[ 1 102 M H2SO 4 ~ . ' " ' " " - . . %

~ : " x \ \ - - - 5mV/seCmv/sec ~ 1

/ " '-~~ . ....... 0.5 mV/sec

c) " ' I mV//s e c I " I I

": - - - 1-104 MSO2+ 5 1G2M H2SO 4 ", .. - - -- 2.1(~4 MS02 + 1.10-2 M H2 S04

',. / N ; - --'-- 5104 M $02+1'102MH2S04

~: \ ....... 5~6a u s02+~ 1~2M.2s04 / - \

50 / ,5 . - . �9

0 I -05 ~1.4 POTE,,,A,(V/ssE)

Fig. 11. Influence of the sweep rate at various points on the corrosion diagram (a and b) and the effect of the total S02 con- centration for u sweep rate of 1 mV/sec on the shape of the voltammetric curves.

Conclusions The effect of HSO3- and pH on corrosion in general,

and on pi t t ing corrosion in particular, of Alloy 600 in solutions containing SO~ and H2SO4 has been studied by cyclic vol tammetry. The electrochemical measure- ments were performed close to the mixed potent ial in the activation range as a funct ion of sweep rate, reIa- tive concentrat ions of HSOs- and SO2 in the solution, and pH.

On the basis of these results, a competit ive adsorp- t ion model involving HSO~- and SO2 or its in te rmedi - ate species has been proposed. The adsorption states are dependent on the relat ive concentrat ion of HSOs- and SOs in the solution and pH. According to the model, the adsorption of the HSO3- anions give rise to breakdown of the oxide film and . tha t of SO2 provides a protective effect. The mechanism of adsorption and the breakdown of the passive film could be studied by the electrode impedance analysis technique.

On the [H2SO4] vs. [SO2tot] corrosion diagram, three areas related to the various surface adsorption states can be distinguished; the first one, at low total concentrat ions of SO2 and H S Q - , is characterized by an oxide film which is sl ightly affected by a pH less than 2; the second one, at in termediate concentrations, is characterized by an oxide film that is strongly affected by the adsorption of HSO3- since acidification below pH 2 destroys the passive state; and the third one, at high concentrations, is characterized by the adsorption of SO2, which protects the metal at more acid pH values than that pH. In this region, the pH decreases as the SO2 concentrat ion increases.

In the first two areas, two types of pi t t ing were ob- served on Alloy 600 at different potentials in the ac- t ivat ion range (--0.47 and --0.52 V/SSE). Their differ- ent appearances can be related to the adsorption state on the oxide film; re la t ively few and deep pits occur at low HSO3- concentrations, while numerous and shallow pits develop at higher HSO3- concentrations, close to the actual concentrat ions of SO2. The first case corresponds to the existence of a lower number of HSO3- adsorption sites in the presence of a low SO2 concentration, while the second case be related to a larger n u m b e r of sites in the presence of higher SO2 concentration.

) unless CC License in place (see abstract).  ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 130.203.136.75Downloaded on 2016-09-17 to IP

2200 J. Electrochem. Soc.: ELECTROCHEMICAL SCIENCE AND TECHNOLOGY October 1982

Fig. 12. Scanning microscope observations of pits obtained. Region 1: 10-4M SO~ -~ 10-1M H2SO4 (A, C); region 2: 10-8M SO~ ~- 9- I0-8M H2S04 (B, D).

Acknowledgment The authors wish to thank Dr. J. R. Donati, head of

the Materials Department of EDF for critical reading of the manuscript and valuable comments.

Manuscript submitted Oct. 6, 1981; revised manu- script received April 5, 1982.

Any discussion of this paper will appear in a Discus- sion Section to be published in the June 1983 JOURNAL. All discussions for the June 1983 Discussion Section should be submitted by Feb. 1, 1983.

Publication costs o~ this article were assisted by the Materials Department o~ Electricit~ de France.

REFERENCES 1. R. J. S. Thompson, Gas J., 224, 608 (1944). 2. K. Barton and E. Beranek, Werkst. Korros., 10, 377

(1959). 3. I. Geld and F. J. D'Oria, Mater. Protection, 5, 16

(1966).

4. K. Bohnenkamp, Werkst. Korros., 19, 792 (1968). 5. R. Ericsson and Sydberger, ibid., 31, 455 (1980). 6. H. T. Shirley and J. E. Truman, J. Iron Steel Inst.,

160, 367 (1948). 7. M. Masuo, T. Tanigawa, Y. Ono, and N. Ohashi,

Boshoku Gijutsu, 27, 565 (1978). 8. G. Herbsleb and W. Schwenk, Werkst. Korros., 17,

745 (1966). 9. E. Pouillard and Chiola, Rev. M~tall., 65, 599 (1968).

10. K. Ogura and N. Takesue, Corrosion, 36, 487 (1980). 11. A. B. Ijzermans and A. J. Van Der Krogt, Corros.

Sci., 8, 679 (1968). 12. A. B. Ijzermans, ibid., 10, 607 (1970). 13. Z. Szklarska-Smialowska, ibid., 18, 97 (1978). 14. E. T. Seo and D. T. Sawyer, Electrochim. Acta, 10,

239 (1965). 15. T. Loucka, J. Electroanal. Chem. Interracial Elec-

trochem., 31, 319 (1971). 16. A. Q. Contractor and Hira Lal, ibid., 93, 99 (1978). 17. "Atlas d'~quilibres ~lectrochimiques," p. 747, ~d.

Gauthier-Villars, Paris (1963).

) unless CC License in place (see abstract).  ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 130.203.136.75Downloaded on 2016-09-17 to IP